Why does my smoke domain stretch the simulation?
When resizing any object relating to a simulation (domain, flow, rigid body, etc.) you should always Ctrl+A, Apply Scale to ensure Blender is aware of it's new dimensions (actually, whenever you rescale anything it's generally a good idea to Apply Scale anyway - uneven scale can cause all kinds of problems with simulations, texturing, etc.). In your case, you should just need to Apply Scale and then re-bake the simulation.